1. Field of the Invention
The invention relates to apparatus for a digital averaging filter, particularly suited for use with aeronautical navigation receivers, for reducing oscillatory deviation errors that occur as the result of reflected incoming radio-navigation signals and/or interfering radio signals.
2. Description of the Prior Art
Currently, a number of different aeronautical radio-navigation systems are in use for enabling a pilot to place his aircraft on a pre-selected path to or from a ground based radio-navigation facility. These systems include both enroute aids, such as very high frequency omnirange (VOR) and TACAN receivers, and landing aids, such as instrument landing system (ILS) and microwave landing system (MLS) receivers.
Enroute aids permit the pilot to fly a direct course to or from a fixed location on the ground. However, an enroute aid, being designed for relatively long distance navigation, only provides the pilot with information that indicates whether the aircraft is heading in a horizontal direction that deviates from a selected course heading to or from a pre-selected ground location, which is usually the location of the transmitter of the enroute aid. As such, enroute aids provide no vertical guidance information. Nonetheless, VOR and TACAN receivers are usually able to successfully guide an aircraft within one or two degrees of the selected course.
Landing aids permit the pilot to fly his aircraft on a proper inbound course and at the right angle of descent to the centerline of an appropriately instrumented runway. As contrasted with enroute aids, landing aids provide both horizontal and vertical guidance information. Here, horizontal guidance information is provided which indicates whether the aircraft is heading in a direction that horizontally deviates to the left or right of a selected inbound course to the runway. In addition, vertical guidance information is provided which indicates whether the aircraft is located above or below a selected inbound glide path extending diagonally outward and upward from a far end of the instrumented runway. Landing aids are generally able to horizontally guide an aircraft to within 30 to 40 feet of the centerline of the runway and vertically guide that aircraft to within 6 to 10 feet of the proper height above the runway threshold.
Deviation information (both course and glide slope) is frequently displayed on zero center deviation meters (commonly referred to as course deviation meters or CDIs) or the like and is also usually fed as an input signal to appropriate automatic navigation (auto-pilot) systems existing onboard the aircraft. In the absence of any interference, radio-navigation aids are quite accurate as noted above.
Unfortunately, these radio-navigation aids are quite susceptible to interference which adversely affects their accuracy. One common cause of interference is reflections. Specifically, radio-navigation signals reflect off objects such as bridges, buildings, power lines, trees, bodies of water, rough terrain and the like. Consequently, radio-navigation signals will arrive at the antenna of the aircraft receiver via two basic paths: signals that directly emanate from the transmitter of the radio-navigation aid and impinge on the antenna, and signals that emanate from the transmitter but reflect off an object before arriving at the antenna. Since a reflected signal always travels via a longer path to the antenna of an aeronautical receiver than a direct signal, a reflected signal arrives at the antenna with a different phase than that the direct signal. Inasmuch as the direct and reflected signals appearing at the antenna combine by vector addition, the reflected signal corrupts the direct signals and injects an erroneous deviation error into the reading produced by the receiver. This error is commonly referred to as "multi-path" error.
To complicate matters, as the aircraft moves along its course, the path length of the reflected signals changes which, in turn, changes the relative phase difference between the direct and reflected signals arriving at the antenna. This, in turn, causes the deviation reading to oscillate back and forth. These oscillations can typically occur with a period ranging between 0.5 seconds to several minutes (although rarely in excess of two seconds) depending on the speed of the aircraft; the angle subtended by the reflecting object and the aircraft with respect to the antenna of the radio-navigation transmitter; the distances between the antenna, the aircraft and the reflecting object; and several other factors.
Disadvantageously, almost every locality contains objects which reflect radio-navigation signals. Inasmuch as these objects are essentially randomly distributed throughout an area, a deviation reading seldom oscillates with a steady period and seldom with exactly the same amplitude on either side of the zero reading. Consequently, experienced pilots generally ignore any rapid excursions in deviation information and instead take corrective action only when they perceive that the average deviation reading is non-zero. Unfortunately, this requires that a pilot constantly watch a CDI needle over a period of time during which he mentally estimates an average reading to yield an assessment of true course deviation. This task is quite burdensome on the pilot and hence quite tiring. Moreover, such mental estimations are often inaccurate.
Not only are radio-navigation signals reflected from ground based objects, these signals are also reflected from other aircraft. Multipath errors due to aircraft reflections are becoming more prevalent as airports become more congested particularly with large aircraft. The Federal Aviation Administration (FAA) requires that a set amount of space must exist between any two aircraft, particularly those inbound to an ILS equipped runway, such that a landing aircraft is well clear of the runway and ground navigation transmitters before another aircraft is allowed to make an ILS approach. As a result of this spacing, aircraft induced oscillatory multipath errors usually possess a relatively short period. However, because these errors are quite unpredictable, they can cause erratic operation of an airborne automatic navigation system as it undertakes an automatic ILS approach. In fact, aircraft induced multipath errors have been known to cause an automatic navigation system to disadvantageously disconnect during an ILS approach. Unfortunately, glide path receivers are particular sensitive to these multipath errors.
Moreover, the frequency band of VOR radio navigation aids (108-118 mHz) lies between that of the FM broadcast band (88-108 mHz) and 2 meter amateur radio band (144-148 mHz). In particular, VOR transmitters operate on 40 channels spaced 50 kHz apart within the band 108-112 mHz and on 120 channels also spaced 50 kHz apart between 112-118 mHz. In addition, VOR transmitters have a relatively low output power, typically 25-200 watts. As such, strong radio signals often exist that locally interfere with VOR signals. This interference, when it occurs, disadvantageously produces erroneous oscillatory deviation errors. The period of such interference induced oscillations generally falls within the same range as that of reflection induced oscillations. The length of this period depends upon many factors, such as the strength of the interfering signal, the difference in frequency between the interfering signal and the particular VOR station then being received, the proximity of the aircraft to the location of the interfering transmitter and the course, speed and altitude of the aircraft relative to the transmitter location. Consequently, the pilot must also be able to recognize and then ignore erroneous oscillatory deviation errors caused by radio interference in addition to those produced by reflections.
Therefore, a need exists in the art for apparatus particularly suited for use in conjunction with aeronautical navigation receivers for reducing oscillatory deviation errors that occur as the result of reflected incoming radio-navigation signals and interfering radio signals.